The utilization of highly dispersed multi-walled carbon nanotubes (MWCNTs) in cementitious materials has shown to substantially improve the mechanical and other properties of the cementitious matrix. For example, copending patent application U.S. Ser. No. 12/322,842 filed Feb. 6, 2009, discloses that the utilization of highly dispersed carbon nanotubes (CNTs) in cementitious materials substantially improves the performance of the cementitious matrix. In particular, by adding a very low amount of MWCNTs or carbon nanofibers (CNFs), at concentrations of 0.025 wt. % to 0.08 wt. % of cement, the strength and stiffness of cement beams increases significantly [U.S. Ser. No. 12/322,842 and references 1-6]. The application of low concentration of MWCNTs and CNFs enables the control of matrix cracks at the nanoscale level [reference 7]. Also, the cost of CNTs at such low concentrations is comparable or lower than that of conventional reinforcement which makes the introduction of CNTs in concrete economically feasible. In addition to the benefits of reinforcement, autogenous shrinkage tests have demonstrated that MWCNTs can also have beneficial effects on the early age strain capacity of cementitious materials, which leads to improved durability of the cement matrix [reference 1].
The current preparation method of MWCNT suspensions for use in cementitious materials includes a one step technique involving the application of ultrasonic energy and the use of a commercially available surfactant to disperse the MWCNTs in the mixing water prior to their addition to cement [U.S. Ser. No. 12/322,842 and references 2, 8]. However, in order to have widespread use of MWCNT-cement nanocomposites, there is a need to produce MWCNT suspensions in large scale production for full-scale application in concrete to decrease the transportation and storage cost of the large volume suspensions for this application.
A number of solution-phase processes exist where carbon nanomaterials, such as CNTs and graphene flakes, are concentrated by the removal of their solvent. This can be achieved by precipitation via addition of organic solvent and vacuum filteration [9], solvent exchange utilizing polymer-organic solvent [10] and sedimentation and decantation by ultracentrifugation [11]. Among these processes, the ultracentrifugation method is ideal due to its simplicity and also for applications where the introduction of organic solvents will become a hindrance. Ultracentrifugation process has been proven as a facile method to increase the concentration of CNTs in aqueous solutions prior to being used in a technique called density gradient ultracentrifugation (DGU). DGU is a solution phase purifying technique that is widely used to separate various forms of carbon nanomaterials by their physical and electronic structures, which depend on the subtle buoyant densities of different species [12-16]. In this technique, the materials of interest are suspended in an aqueous solution and layered within a density gradient, thus their high initial concentration is essential for the optimal yield of separation after ultracentrifugation. To address this issue, a preparative ultracentrifugation process called pelleting, which is generally used to sediment solidified organic compounds out of solutions, has been adapted from biology [17-18]. During this technique, nanomaterials in aqueous suspensions are presented under a centrifugal force inside a tube and travel towards the bottom at certain sedimentation rate, forming a highly concentrated region which can be recovered after decantation.